The cleanup of acid gasses or sour gas, such as CO2 in particular, from natural gas and in oil refining has been an extensively practiced technology. The industrial removal of CO2 from natural gas dates back to the 1930's. In the 21st century, due to the potential impact of anthropogenic CO2 emissions on the climate, post-combustion CO2 capture has gained tremendous attention. While several technologies exist for the removal of acid gasses, one of the most commonly employed practices is the use of aqueous amines. Of these amines, tertiary amines are often used for natural gas applications due to their low energy of regeneration. For post-combustion CO2 capture applications primary and secondary amines tend to be in part favored by their faster rate at the low driving force condition. Regardless of the application, the mass transfer rate in the absorber column dictates the size of the column (capital cost) used and, consequently, has a substantial impact on the overall process cost. An overall process depicting a thermal swing process is presented in FIG. 1. An aqueous amine solution is circulated between the absorber 10 and stripper 12. The CO2 containing gas enters the bottom of the absorber where it contacts the aqueous amine absorbent removing it from the gas stream. The liquid solution, CO2 rich amine solution, is then passed through a heat exchanger 14 to improve efficiency before being heated to a higher temperature in the stripper 12. The stripper 12 removes the CO2 as a gas from the amine solution to produce a lean, or CO2 deficient solution. The lean solution is returned to the absorber 10 by way of the heat exchanger to repeat the process.
In order to minimize system capital (absorber cost) it is important to maximize the overall mass transfer rate for the scrubber system as there is a direct correlation between the two. Primary (RNH2) and secondary (R2NH) amines are capable of achieving a high mass transfer rate per unit due to the direct chemical reaction with CO2 to form a carbamate. However, they show a lower than desirable capacity. It requires 2 moles of amine to capture 1 mole of CO2 since the carbamate is then negatively charged another amine must absorb the proton formed. Furthermore, due to the high enthalpy of absorption from the formation of the carbamate the regeneration energy is high. Tertiary amines (R3N) show a significant decrease in mass transfer rates due to the inability to react with CO2 directly; however, they show significantly lower heats of regeneration. The described process seeks to exploit the advantages of both systems without inheriting the limitations exhibited by either. More specifically, by promoting the CO2 capture reaction rate the absorber will be smaller and reduce the process capital. The intrinsic lower energy of regeneration for the tertiary amine solvent minimizes operating costs.
While gas sweetening applications represent the most immediate opportunity for application of the described invention, post-combustion CO2 capture could represent a large potential application of the described technology. The market driver for this application will be the regulation of CO2 emissions due to concern about its environmental impact towards global climate change or a need for CO2 for utilization purposes such as enhanced oil recovery (EOR). Carbon dioxide capture and sequestration (CCS) from large stationary sources such as fossil fuel combusting electricity generators represents one method to reduce the increase in atmospheric CO2 levels. The challenges of post-combustion CO2 capture include the fact that flue gas from utility boilers is at near atmospheric pressure and the concentration of CO2 in the flue gas is relatively low at 12-14%. Another technical hurdle is the energy requirements for the CO2 capture/desorption devices to regenerate absorber reagents. Generally speaking, the energy required for CO2 capture and sequestration using MEA is estimated to reduce a PC plant's output by about 30 percent, which equates to a very substantial 60-80% increase in the cost of electricity. The ability to capture and store CO2 more efficiently will be highly valued by utilities.
In broad terms the described method seeks to add a promoter amine, in the form of a substituted primary and/or secondary amine to a tertiary amine to form a solvent. The promoters serve to increase the overall mass transfer of the acid gas into the absorption solvent. The promoters are designed to have particularly low volatility without contributing significant viscosity to the solution. The described promoters achieve this attribute without being an ionic compound which can negatively impact mass transfer. The low volatility and low viscosity are achieved by using alternate functional groups in addition to the amine functional group that reacts with the CO2 molecule. Low volatility is important to reduce amine loss in the CO2 capture process. This property is often achieved by using alcohol groups in addition to the amine group. However, the alcohol groups are hydrogen bond donors which add more solution viscosity due to intermolecular bonding.